This application claims the benefit of China patent application Serial No. 98120652.2, filed on Oct. 21, 1998, and entitled xe2x80x9cMicrobolom Infared Sensors,xe2x80x9d and naming Xiang Zheng Tu as inventor.
1. Field of the Invention
The invention relates to a microbolometer infrared sensor. Particularly, the invention relates to a microbolometer infrared sensor utilizing a porous silicon bridge as its thermal isolating and mechanical supporting structure.
2. Description of the Related Art
Microbolometer infrared sensors have a wide variety of applications, ranging from security night sights and driving aids to area surveillance, fire-fighting, industrial radiometry, search and rescue, border patrol and vehicle collision avoidance.
Newly developed microbolometer infrared sensors are fabricated based on integrated circuit fabricating techniques and micromachining techniques. This type of microbolometer infrared sensors can operate at room temperature without the need for cryocooling, which improves sensor reliability, reduces sensor power and cost and therefore gains a variety of civilian or consumer applications.
Generally, a microbolometer infrared sensor comprises a temperature-dependent resistor and an infrared absorber. To achieve a high responsively of the microbolometer infrared sensor, there are two methods to be commonly adopted. One is to choose large resistance temperature coefficient of the resistor material, so that a small increase in temperature gives rise to a significant change in the resistance of the temperature-dependent resistor. The other is to reduce the thermal exchange between the temperature-dependent resistor and its surroundings, so that most of the thermal energy absorbed by the infrared absorber contribute to raise the temperature of the resistor. This can be accomplished by minimizing the thermal contact by suspending the resistor in air or vacuum.
As is known in the art, micromachining technologies are being used to form various microstructures. One such microstructure is a microbrigde anchored to a silicon substrate through thin supports which can be used to built a microbolometer infrared sensor. The geometry and the thermal conductivity of the microstructure determine the thermal insulation. Typical microstructure materials are silicon oxide or silicon nitride.
Moreover, an array of such microbolometer infrared sensors is formed integrally with an integrated circuit. The integrated circuit is used to develop signals produced by the microbolometer infrared sensors in response to the infrared energy impinging on to the array thereof.
One type of microbolometer infrared sensors utilizes thermalgrown silicon dioxide as a microstructure material. Residual stress in a thermal-grown silicon dioxide film is up to 200 MPa. This limits the thickness of a silicon dioxide film to be less than 2 xcexc. Actually, the thickness of silicon dioxide films used in standard integrated circuits is about 1 xcexc. Microstructures of such thin silicon dioxide films have very poor mechanical properties.
Another type of microbolometer infrared sensors utilizes silicon nitride as a microstructure material. Although silicon-rich silicon nitride films formed by low-pressure chemical vapor deposition (LPCVD) have lower stress, but its thickness is still limited to about 2 xcexc. In addition to mechanical strength problem, microstructures of silicon nitride also have following other problems.
1. Patterning a silicon nitride film formed on the surface of a silicon substrate creates steps on the surface of the substrate. Thicker the silicon nitride films higher the steps. This is not compatible with standard semiconductor technologies.
2. The thermal conductivity of silicon nitride is not quite low. It is not suitable for the use as a thermal insulating material.
Common problems with microbolometer infrared sensors based on silicon oxide or silicon nitride are not only to have poor mechanical strength, but also to have difficulties with processing. xe2x80x9cMicroelectronics first and micromachining lastxe2x80x9d is an important strategy for fabricating micro-electrical-mechanical system (MEMS) devices. As one of MEMS devices, it is a hope to fabricate microbolometer infrared sensors after processing the integrated circuitry necessary for the driving and read-out electronics. But forming the silicon oxide films or the silicon nitride films are not low temperature processes. It can not be performed after processing the integrated circuits.
In view of the above problems, the present invention provides a microbolometer infrared sensor having the following features.
One feature of the microbolometer infrared sensor provided by the present invention is to form a microbridge and its supports in a silicon substrate so that there is no steps created on the surface of the substrate, and therefor no influence on the planar processing.
Another feature of the microbolometer infrared sensor provided by the present invention is that the microbridge and its supports have higher mechanical strength and lower residual stress.
Still another feature of the microbolometer infrared sensor provided by the present invention is that the microbridge and its supports have lower thermal conductance or higher thermal resistance.
Still another feature of the microbolometer infrared sensor provided by the present invention is to form the microbridge and its supports at lower temperature so that it can be done after processing the integrated circuit.
Still another feature of the microbolometer infrared sensor provided by the present invention is that the microbridge and its supports are separated from the substrate by a narrower gap so that they are forced to touch the substrate without damage.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings.